Ash and friends are travelling to Castelia City so Ash can battle the Castelia Gym Leader. While in Pinwheel Forest, they encounter a Sewaddle who swung right in and attack Pikachu with Bug Bite. As Ash followed Sewaddle through the forest in order to catch it (since he experience it's moves), they meet Burgh, the Castelia Gym Leader.
This is the first time Ash catches a Pokemon with a team of six since catching Krabby early in Kanto.
This is the first time Ash switches his team, swapping Sewaddle with Pidove.
If a Trainer catches a Pokemon with a team of six, the Poke Ball seals itself unless one of the Trainer's Pokemon is sent to the Professor's lab.